1. Field of the Invention
This invention relates to improvements in a character display employing a cathode ray tube (CRT).
2. Description of the Prior Art
In conventional CRT character displays, it is customary that the size of a character to be displayed is predetermined and cannot be changed at will. It would be convenient if the size of the character to be diplayed could be altered in accordance with the number of characters or the importance of the content to be displayed. One possible method of having character size is to change the scanning speed and the amount of vertical deflection of a beam of the CRT display. In principle, the size of the character can be altered in this manner, but in practice, it is difficult to change the scanning speed and the amount of vertical deflection of the beam.
It has been suggested, in the case of displaying a character by reading out a dot storage pattern from a character generator, that the size of the character to be displayed be altered by exchanging the dot storage pattern with a desired one; but this necessitates the modification of stored data and the modification is not easy.
FIG. 1 shows in block form a conventional character display and FIG. 2 is explanatory of its operation.
In FIG. 1, reference character RM indicates a refresh memory; CRTC designates a CRT controller; CG identifies a character generator; PSC denotes a parallel-serial converter; CRMC represents a CRT monitor signal control circuit; DOTC shows a dot clock generator; CHC refers to a character clock generator; MPX indicates a multiplexor; AB designates an address bus; DB shows a data bus; PRM identifies a parameter memory; RAS denotes a row address selector; and CAS represents a character address selector.
In the case of providing a display of one picture in which one character is formed with seven bits in a horizontal direction and nine bits in a vertical direction, the following method is employed. In the refresh memory RM there is stored character data to be displayed on the display surface of the cathode ray tube. The addresses of the refresh memory RM correspond to the character positions (defined by rows and columns) on the display surface, and the stored content of the refresh memory RM represents a character to be displayed. The character generator CG has stored therein data indicating the dot positions in rows R1 to R9 for all characters which can be displayed, and when supplied with the character data from the refressh memory RM, that is, a character address CA, and a row address RA from the CRT controller CRTC, the character generator CG provides 7-bit dot position information to the parallel-serial converter PSC.
The number of characters a to be displayed for each line, the number of lines b for each picture and the number of rasters c (the number of rows) for each character are prestored in the parameter memory PRM of the CRT controller CRTC via the data bus DB. The CRT controller CRTC reads out of the refresh memory RM a character to be displayed in a first line to apply the character address CA to the character address selector CAS and, at the same time, provides one pulse signal as address information to the row address selector RAS to retain it in a first row select state. As a consequence, the dot data of a first row of the first line is successively provided to the parallel-serial converter PSC, the output from which is applied to the CRT monitor signal control circuit CRMC in synchronism with a dot clock DOTC in FIG. 2 and used as a video signal VIDEO.
The selection of each character is performed in synchronism with character clock pulses CHC in FIG. 2 and when the count value of the character clock pulses CHC is detected by comparison to match with the number of the character a in the parameter memory PRM, it is confirmed that the display operation of the first row of the first line is completed. At this time, the CRT controller CRTC provides one pulse as row address information to the row address selector RAS to advance the row address by one step, by which the selector RAS is switched to the state of a second row selection, and then the same operations as described above are repeated.
When the count value of the character clock pulses CHC coincides with the number of displayed character a in a ninth row select state, it indicates the completion of the display operation for the first line, and the CRT controller CRTC successively accesses to characters of a second line to read them out of the refresh memory RM. Thereafter the display operation is similarly conducted for each of the b lines, thus completing one scanning and display operation for one picture. In FIG. 1, reference character H indicates the horizontal deflection output and V the vertical deflection output.